const s = Symbol('zhangsan')
s.description = 'lisi' // 无效
console.log(s) // Symbol(zhangsan)

const s1 = Symbol()
const s2 = Symbol()
console.log(s1 === s2) // false

const obj = { name: 'zhangsan' }
const s3 = Symbol(obj) // 对象结构描述会自动 toString()
console.log(s3.description) // [object Object]
